Co-transport of polystyrene nanoplastics and soil colloids in saturated porous media: influence of pH and ionic strength

As emerging pollutants, nanoplastics (NPs) have attracted growing concern owing to their ubiquitous detection in aquatic and terrestrial environments. Nevertheless, few investigations have addressed the co-transport of NPs and colloids within the soil-aquifer system. This study examines the co-transport behavior of polystyrene nanoplastics (PSNPs) and soil colloids in saturated porous media via column breakthrough experiments, with consideration of factors including colloid types, solution pH, and ionic strength (IS). Adsorption tests and the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory were applied to clarify interparticle interaction mechanisms. The results revealed that the co-transport of PSNPs with colloids was influenced by both the carrier capacity of colloids and the stability of the particle system. Soil colloids significantly promoted PSNP mobility by serving as carriers and enhancing PSNPs' stability. Elevated pH intensified interparticle repulsive forces, thereby improving system stability. Although the carrier effect weakened under high pH, colloid-facilitated transport of PSNPs became more evident. Increasing IS compromised both carrier capacity and system stability, substantially reducing PSNP effluent concentrations. Compared to illite colloids, bentonite colloids demonstrated higher carrier capacity and a stronger stabilizing effect on PSNPs under neutral to alkaline conditions, leading to more pronounced facilitation of PSNP transport. This work offers important insights into the transport dynamics of NPs in soil-aquifer environments.